This invention relates to protective coatings and to methods of producing such coatings. The invention relates more particularly to protective coatings for component parts of domestic and catering cooking equipment.
Protection for such parts has, in the past, been provided by a vitreous enamel coating but whilst such coatings effectively protect the base metal of which the part is made for as long as the coating remains intact, it is found that contamination on the coating resulting from cooking operations is difficult to remove. It has been proposed to incorporate in the coating a substance sometimes referred to as an oxidising agent and sometimes as a catalyst which substance promotes the breakdown of such contamination and permits its erradication at temperatures within the range of normal cooking operations. Difficulty is, however, experienced in incorporating the substance in the coating to ensure effective performance of the coating without reducing the resistance of the coating to abrasion and rubbing to which it is sometimes exposed. Effectiveness of the substance is, to some extent, determined by the surface area or surface porosity over which the substance is distributed.
In conventional vitreous enamel coating processes it is found that the surface porosity of the slurry applied to a surface during the coating process is relatively high and remains so during drying and the initial stages of the final heating to fuse the dried slurry. As the sintering temperature is reached, the surface porosity drops rapidly in an uncontrollable manner to a very low value. At this stage the dried slurry has become fully fused and the resultant coating is extremely hard and smooth-surfaced.
For applications such as those referred to above, a hard coating is desirable because it resists abrasion well but it is found that the effectiveness of the substance is reduced, in many cases seriously, by the fully-fired nature of the coating.